[MagicRat]

I assume you have a manual transmission.
I suspect your guess is correct. Your clutch is slipping.

When you step on the gas and add more power to the engine, a bad clutch cannot lock together and starts to slip. The coefficient of friction will decrease and the car does accelerate slower than if the clutch was not slipping at all.
Its a bit like locking up a car's brakes on a wet road. Your car can stop faster if the tires are still turning, than if they are locked up.

Chances are the mechanic just adjusted your clutch linkage last year. When a cluthc starts to slip, it will wear out very quickly. Perhaps your clutch was already worn, or perhaps it started to slip because the linkage was out of adjustment.
I suspect that by now it is too worn to save with just a clutch adjustment, and it needs replacement.
Sure, you can try adjusting it to see if the slipping goes away, but be prepared to pay for a replacement, with is a major repair.